1. Field of the Invention
The present invention generally relates to a digital television receiver, and more particularly, to a demodulation circuit built in a digital television receiver capable of restoring a carrier and a symbol clock, and a demodulation method therefor.
2. Description of the Related Art
In general, due to noise, it is difficult to successfully perform frequency and symbol synchronization using a terrestrial receiving system with multiple paths. Therefore, current research is aimed at improving the receiving performance of terrestrial receiving systems. In particular, research shows that the receiving performance may be improved by increasing the performance of an equalizer. However, prior to improving the equalizer performance, a received signal should be initially demodulated and synchronized with a symbol clock.
FIG. 1 is a block diagram illustrating a conventional digital television receiving system 100. Referring to FIG. 1, when a digital broadcast signal is input to a tuner 101 via an antenna, the tuner 101 selects a desired channel frequency. A signal output from the tuner 101 passes through an intermediate frequency (IF) converter 102. The IF converter 102 generates a signal in an IF band.
The signal output from the IF converter 102, which is output at 44 MHz, is then input to a channel receiver 103 and quantized by direct sampling. Then, the quantized signal passes through a source decoder 104, is input to a display 105, and displayed on a screen.
FIG. 2 is a block diagram illustrating the structure of a channel receiver as shown in FIG. 1. The channel receiver 200 includes an analog-to-digital converter (ADC) 201, a demodulator 202, an equalizer 203, and a forward error corrector (FEC) 204.
The ADC 201 quantizes an IF signal and the demodulator 202 performs timing and frequency synchronization for synchronization of received data.
FIG. 3 is a block diagram illustrating a conventional digital television receiver 300. Referring to FIG. 3, the digital television receiver 300 includes a tuner 301, an SAW filter 302, an ADC 303, a resampler 305, a matched filter 306, a multiplier 307, a timing restoration unit 309, a carrier restoration unit 313, and a channel equalizer 308.
Referring to FIG. 3, when a signal is input to the tuner 301 via an antenna (not shown), the tuner 301 selects a channel frequency and lowers a radio frequency (RF) band of the channel frequency to a fixed IF band. The SAW filter 302 removes all frequency bands, except for a frequency band that carries information, of the signal output from the tuner 301, and outputs the signal at the frequency band to the ADC 303.
The ADC 303 converts the signal output from the SAW filter 302 into a digital signal by sampling the signal at a fixed frequency of 24.69 MHz and outputs the digital signal to the resampler 305 for restoration of a digital symbol clock.
The resampler 305 receives timing errors of symbols, which are obtained from baseband symbol processing, from the timing restoration unit 309 and performs interpolation to reduce an error between the digital signals. As a result of the interpolation, the digital signal sampled at a frequency of 24.69 MHz passes through the resampler 305, and then, the symbol rate of the digital signal is interpolated from n points of the original symbol rate.
The signal output of the resampler 305 is input to the multiplier 307, and the baseband digital signal output from the multiplier 307 passes through the matched filter 306 and is sequentially input to the timing restoration unit 309, the carrier restoration unit 313, and the channel equalizer 308.
The carrier restoration unit 313 removes a carrier frequency offset and phase noise from the baseband digital signal output from the multiplier 307 and feeds a complex sinewave, as the result of removal, back to the multiplier 307.
The timing restoration unit 309 extracts information regarding timing errors from the baseband digital signal output from the multiplier 307 and adjusts the sampling timing of the resamplier 305 based on the extracted information.
The conventional television receiver normally requires a demodulation system to adjust the rate of an error in a received signal caused by the multiple paths of the television receiver. The demodulation system also normally performs frequency synchronization even if a frequency error is large.